[llvm] [libc] [reland][libc][NFC] Refactor FPBits and remove LongDoubleBits specialization (PR #78447)

[Clement Courbet via llvm-commits]

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@@ -171,68 +171,98 @@ struct FPRepBase : public internal::FPLayout<fp_type> {
     return StorageType(1) << position;
   }
 
+  template <typename T> struct Opaque {
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legrosbuffle wrote:

Doc + change the name to something clearer, maybe `TypedInt` ?

https://github.com/llvm/llvm-project/pull/78447